|MadSci Network: Engineering|
I suspect that you might be actually asking this: since an airplane's wing must be longer and more curved on top in order to create aerodynamic lift, how can a plane still fly when it is upside-down? (Am I right? If not, then I'm not answering your question!)
Many textbooks claim that airplane wings create lift like so:
Unfortunately this line of reasoning is wrong. And worse, it widely "infects" textbooks of all kinds. See K-6 Science Textbook Misconceptions under wing.
Also see Airfoil Lifting Force Misconception.
The above "Bernoilli" explanation is wrong in several places. Most importantly, wind-tunnel photographs prove that the air which divides at the leading edge of the wing does not rejoin at the trailing edge. Other problems: thin wings such as Hang Gliders, sailboat sails, paper airplanes, and even the Wright Brothers' flyer all have equal pathlengths above and below the wing, yet these craft can fly.
Also, as you point out, inverted flight is not as impossible as the "Bernoulli" explanation would have you believe. Here is an alternative explanation:
So, how can a plane fly upside-down? The pilot simply flips the plane (or performs half of a loop-the-loop), then uses the control surfaces ("flaps") to tilt the plane so the wing still forces air downwards. But this is what the pilot was doing for right-side-up flight too: tilting the plane so the wing acts downwards upon the air.
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